Apparatus for supported discharge sputter-coating of a substrate

ABSTRACT

A SUPPORTED DISCHARGE SPUTTER COATING APPARATUS IS PROVIDED WITH THE TARGET LOCATED BETWEEN THE CENTER OF A HIGHLY IONIZED PLASMA AND THE SUBSTRATE OT BE COATED.

March 14, 1972 -r EI'AL 3,649,502

APPARATUS FOR SUPPORTED mscmmom SPUTTER-COATING OF A SUBSTRATE Filed Aug. 14, 1969 2 Sheets-Sheet l POWER I SUPPLY TARGET 0 SUPPLY w m m W SUPPORTING RF SUPPLY Wb/W ATTORN EYS March 14, 1972 -r ErAL 3,649,502

APPARATUS FOR SUPPORTED DISCHARGE SPUTTER-COATING OF A SUBSTRATE Filed Aug. 14, 1969 2 Sheets-Sheet 2 F IG...2

0 VOLTAGE SUPPLY 43 TARGET (OPTION'L) SUPPLY /7 A INVENTORS HG 3 LAWRENCE HERTE SHELDON D. SOFTKY BY NICHOLAS D. SCHOENBERGER W ;MM-/

ATTORNEYS 'nited States Calif.

Filed Aug. 14, 1969, Ser. No. 849,950 Int. Cl. C23c /00 U.S. Cl. 204192 10 Claims ABSTRACT OF THE DISCLOSURE A supported discharge sputter coating apparatus is provided with the target located between the center of a highly ionized plasma and the substrate to be coated.

This invention relates to sputter coating thin layers of target material onto the surface of a substrate and more particularly discloses an improved apparatus and method for supported discharge sputter coating.

The deposition of extremely thin films of metal or dielectrics on substrates is often effected by supported discharge sputtering. In this process, a highly ionized plasma is created in a gas at extreme low pressure by either a radio frequency coil or a low voltage are between a hot cathode and plate. Gas ions are drawn out of the plasma to a target of coating material maintained at a negative potential. The ions collide with the coating material of the target dislocating the individual atoms and molecules of the target. These dislocated atoms and molecules impinge at relatively high velocities on the surface of the substrate producing an extremely thin coating of firmly bonded deposits of the coating material of the target on the substrate.

In prior art sputtering processes, the plasma in its supported discharge state is between the target and the substrate to be coated. Ions, attracted from the plasma onto the target material, rebound the dislocated target particles back through the plasma to produce the desired coating of the substrate.

The back-scattered distribution of the target material, inherent in the coating process where the plasma is between the substrate and target, results in an ineflicient production of the sputtered coating material. The rebounding particles of target material are knocked out at relatively low velocities and only small relative yield per ion. Moreover, these rebounding particles have poor directional dispersion; they frequently miss the substrate and contaminate other surfaces within the coating apparatus.

Furthermore, the spatial separation of the back-scattered coating required to support sufficient plasma between the target and substrate results in a relatively low solid angle for acceptance of the sputtered coating material on the substrate. Moreover, this spatial separation, commonly in excess of 0.8 of an inch, allows increased opportunity for particle collision of the coating material with the gas when passing from the target to the substrate.

Finally, the substrate in such apparatus is exposed directly to the plasma. Such exposure results in the substrate being heated by bombardment with primary electrons, secondary electrons and ions. Further, the substrate is exposed directly to radio frequency heating and infrared heating of the gas.

An object of this invention is to provide a forward sputtering target bombardment. In the improved apparatus and method herein described, a target is disposed between the center of the highly ionized plasma and the 3,649,592 Patented Mar. 14, 1972 substrate. Ions bombard the target at a small angle of incidence traveling in a direction which is towards the substrate. This angle and direction of ion collision causes a transfer of momentum to the particles of target material in the direction of the substrate resulting in a greater target material yield per unit of ion current flow, and hence, a greater efficiency of deposition.

An advantage of this forward sputtering of the target material is that the dislocated particles of the target impact the substrate at a higher velocity providing improved coating adhesion.

An additional advantage of the forward sputtering" collision is that the dislocated particles of target material are focused onto the target with greater precision, greatly reducing the possibilities of contaminating other surfaces within the coating apparatus.

A further object of this invention is to reduce the spatial interval between the target and substrate to a distance below 0.25 inch by relocating the ionized plasma so that the target is between the plasma and substrate.

An advantage of the reduced spatial interval between the target and substrate is that a high solid angle for acceptance of the sputtered material from the target to the substrate is achieved.

A further advantage of the reduced spatial interval between the target and substrate is that the dislocated particles of coating target material travel a shorter path to the substrate. This shorter path subjects the particles to a minimum probability of collision with other particles within the chamber.

A further object of this invention is to shield the substrate from the plasma by disposing the target between the plasma and substrate.

An advantage of this target position is that it reduces the probability of ions, primary electrons, and secondary electrons of the plasma impacting the surface of the substrate and producing heating thereof.

A further advantage of the shield provided by the interposed target is that it absorbs both the radio frequency energy and infrared energy of the plasma in relatively large quantities, retarding these energies produced within the plasma from producing a detrimental heating effect on the substrate and coating.

A still further object of this invention is to disclose a target grid which will accommodate the forward sputtering of this invention. Preferably, the target grid includes a series of interconnected rods, these rods flattened in the plane of the shortest distance between the plasma and substrate. As flattened, the individual rods or members of the grids accommodate ion collisions at a near tangent or steep angle of incidence permitting the transfer of momentum to occur through the target with greater etficiency of deposition.

Yet another object of this invention is to provide the target with a shield which will arrest those ion collisions which will cause the dislocated target material to miss the substrate and coat and contaminate other surfaces interior of the coating apparatus.

An advantage of this overlying shield of the improved target is that the ions are constrained to follow electric field lines extending between either plasma or target shield and the target. These field lines results in a highly directionalized impact of the ions at surfaces of the target where their impact occurs at a steep angle of incidence.

Other objects, features and advantages of this invention will be more apparent after referring to the attached drawing in which:

FIG. 1 is a schematic of the apparatus for producing the deposition of materials by means of the forward sputtering supported gas discharge of this invention;

FIG. 2. is a side elevation section of the plasma, target and substrate illustrated in FIG. 1; and,

FIG. 3 is a side elevation section similar to FIG. 2 illustrating an alternative embodiment of the target.

With reference to FIG. 1, chamber A, typically of glass or other transparent material, is shown connected to a vacuum gas system B. Vacuum gas system B provides interior of chamber A low pressure gas atmosphere in a range below microns of mercury. Ionizing apparatus C for the gas, here shown as an induction coil delivering radio frequency energy, causes the gas interior of chamber A to contain a highly ionized plasma P independent of any potential between the target and substrate. Target D, maintained at a negative potential with respect to plasma P, is bombarded with high velocity positive ions from the plasma. These ions dislodge individual particles of the target material causing them to deposit out on substrate E on revolving cylindrical substrate table F.

Preferably the gas interior of chamber A is of heavy atomic weight, inert and at a pressure less than 10 microns of mercury. Argon is typically used. While gases of light atomic weight, such as helium and neon can be used, it has been found that the mass of their respective ions is insufficient to provide efficient particle dislocation. Usually, the gas is inert so as to prevent undesired chemical reactions with the target, substrate and other materials interior of the chamber. However, chemically active gases such as hydrogen, oxygen and the like can be used.

Ionizing apparatus C can be any apparatus sufiicient for maintaining a supported glow in the gas interior of chamber A. Such apparatuses include the radio frequency coil here illustrated which is powered by a supporting radio frequency power supply at a frequentcy of 13.5 mHz. at 1.2 kw. and 50 ohms.

Target D must be a vacuum compatible material; it must not have contained components such as gas vapors, which will destroy the purity of the low gas vapor pressure maintained interior of chamber A. As the material of the target is subjected to relatively high heating from primary and secondary electron bombardment, ion bombardment, infrared radiation and radio frequency radiation, it is preferably water cooled through piping 17.

Target D is maintained at a negative potential with respect to plasma P in the range of 300 to 2,000 volts. As illustrated in FIG. 1, such negative voltage potential can be maintained by connection of the target to direct current power supply 20, when the target material is a metal only. Alternately, such a negative voltage potential can be maintained by connection of the target to radio frequency power supply 22 in series through blocking capacitor 23, when target D is either a metal or dielectric. As both illustrated methods of maintaining target D at its negative potential are well understood in the prior art, they will not be discussed further herein. It should be noted, however, that virtually, any apparatus which maintains the target at the desired negative potential with respect to plasma P is sufficient for the practice of this invention.

With reference to FIG. 2, it can be seen that the grid of target D is composed of a plurality of individual rods 25 disposed between the center of plasma P and the surface of substrate E. These rods can either be placed in parallel side-by-side relation or alternately can intersect one another to form a network of crossing members.

Preferably rods 25 are flattened parallel to the shortest distance between the plasma and substrate. While the grid of the target D could be made of individual rounded or square rods, such construction is not preferred. Such rounded or square rods would fail to expose the bulk of the surface area of the target material substantially parallel to the shortest path between plasma P and substrate E. This substantially non-parallel disposition would result in ion collisions with the target material which would disperse the dislodged particles along paths which 4 would not impact the substrate E and result in contamination of other surfaces interior of the coating apparatus.

Substrate E is subject to the same requirement as the material of the target D, i.e., it is a vacuum compatible material lacking in atoms or compounds of a vapor pressure that would destroy the low gas vapor pressure maintained interior of chamber A. Typically, this substrate is placed overlying a substrate table F, which table is shown in FIG. 1 as a roller 30. The substrate table is maintained at a ground potential.

Due to the grid shaped configuration of target D the dislodged or sputtered atoms of the target coating substrate E can be laid down in uneven patterns. To achieve an even and uniform coating of the substrate -E, table F typically is rotated so that each unit of substrate surface is exposed to a uniform coating bombardment during the coating process. Additionally, such a roller provides for periodic exposure of segments of the substrate surface to the bombarding atoms, allowing the non-exposed increments of the substrate time to cool.

With reference to FIG. 3, an alternate target D is illustrated interposed between plasma P and substrate E. Target D includes a first grid 42 and a second grid 43 with a small spatial interval therebetween. Similar to the grid construction of target D previously illustrated, grids 42 and 43 can either comprise rods in side-by-side relation electrically communicated or alternately crossed and connected members.

The rods of grid 42 are positioned and dimensioned so as to immediately overlie the rods of grid 43. These rods are here shown elliptical in cross section with the major axis of the ellipse normal to the shortest distance between plasma P and substrate E.

Typically, grid 42 is maintained at a neutral electrical potential with respect to the plasma P. Such a potential can be created by either immersing the grid within the plasma so that it is electrically floated with respect thereto or alternately attaching the grid to an external voltage supply which is adjusted to provide the desired neutral plasma potential.

Underlying grid 42 at a short spatial interval less than the Crooks dark space for the plasma P, grid 43 is located. Grid 43 is the target for the ion collisions.

The individual rods 43 are complementary in plan to the overlying grid 42; as previously stated, these rods can either be electrically communicated rods in side-byside relation or alternately crossed and connected members.

The rods of grid 43 are here illustrated wedge-shaped in cross section having the flattened and enlarged portion of the wedge underlying the elliptical cross section of the rods of grid 42. Typically, the apex of the wedge extends away from overlying grid 42 towards substrate E. Grid 43 of target D, as in the case of rods 25 of target D, is connected to a voltage supply that maintains a 300 to 2,000 negative voltage potential on its surfaces.

It will be noted that grid 42 will not be bombarded with ions of the plasma to any appreciable degree. This is true since the potential of grid 42 with respect to the plasma P is neutral and consequently no accelerating field exists to attract ions to this grid. As this grid immediately overlies the flattened and enlarged portion of the wedge-shaped members of the grid 43, it will shield this portion of grid 43 from bombardment. The plasma, thus, will not be wastefully attracted to the upper leading edge of the grid 43 where the sputtering yield of the ion impact cannot reach the substrate and can only coat surfaces interior of the chamber contaminating the system.

A second advantage of the grids 42 and 43 of target D is that the ions will be constrained to follow electric field lines of maximum potential drop between the plasma P and the lower converging surfaces of the wedge-shaped cross section of the members of grid 43. These electric field lines, illustrated in FIG. 2, extend from the plasma P curving inwardly towards the lower and converging surfaces of the target grid 43 having a normal incidence with these surfaces.

Typically, ions will be attracted to target D along the electric field lines 50 until immediately before their impact with the downwardly converging sidewalls of grid 43. Immediately prior to such impact, the momentum of the ions will cause a departure from the path described by the electric field lines 50 and will cause an ion impact on the target D" at a steep angle of incidence. This steep angle of incidence will produce a large dislocation of target material and will result in a directional transfer of momentum with resulting higher velocity and desirable direction of the particles of coating target material. Consequently, all the advantages previously illustrated with respect to target D will be enhanced; namely, the rate of sputtering deposition will be greater, this deposition will be strongly focused towards the substrate, and the sputtered material will have a higher velocity and improved adherence.

These and other modifications of our invention may be practiced, it being understood that the form of our invention as described above is to be taken as a preferred example of the same. Such description has been by way of illustration and example for purposes of clarity and understanding. Changes and modifications may be made without departing from the spirit of our invention.

We claim:

1. Apparatus for sputter coating a substrate, comprising: a sealed chamber; means for maintaining a gas within said chamber with a vapor pressure below microns of mercury; means for ionizing a portion of said gas within said chamber to highly ionized plasma; means for disposing said substrate adjacent said plasma; a target grid of coating material disposed between the center of said plasma and said substrate, the grid including a plurality of members each having flattened sides extending substantially in the direction of the shortest distance between said plasma and substrate; and means for maintaining said target at a negative voltage potential.

2. Apparatus according to claim 1 wherein the fiattened sides comprise a plurality of surfaces extending over substantially the full thickness of the target grid in the direction of the shortest path between the plasma and the substrate.

3. Apparaus for sputter-coating a substrate comprising: a sealed chamber; means for maintaining low pressure gas within the chamber; means for ionizing at least a portion of the gas within the chamber to form an ionized plasma; means for disposing the substrate adjacent the plasma; a first target grid of the coating material disposed between the center of the plasma and the substrate; a second grid disposed between the first grid and the center of the plasma; individual members of the sec ond grid each overlying an individual member of the first grid to maintain passages from the plasma to the substrate and shield the first grid from the plasma; means for maintaining the second grid at the electrical potential of the plasma; and means for maintaining the first grid at a negative electrical potential with respect to the potential of the plasma.

4. The apparatus of claim 3 and wherein, said means for maintaining said second grid at potential of said plasma includes disposing said grid within said plasma.

5. The apparatus of claim 3 and wherein, said means for maintaining said second grid at the electrical potential of said plasma includes a voltage supply connected to said second grid, said voltage supply energizing said second grid to the potential of said plasma.

6. Apparatus for sputter coating comprising: a sealed chamber; means for maintaining a gas within said sealed chamber at a pressure below 10 microns of mercury; means for exciting a portion of said gas in said chamber to a highly ionized plasma; means for supporting a substrate within said chamber adjacent to said highly ionized plasma; a target of coating material including means forming passages from the plasma to the substrate, the target being disposed between the center of said highly ionized plasma and said substrate; means for electrically shielding said target from substantially normal impact of ions from said plasma; and means for energizing said target of coating material at a negative potential in the range of 300 to 2,000 volts.

7. The apparatus according to claim 6 and wherein, means for providing electric field lines to the surface of said target are provided between the center of said plasma and said target.

8. A process for sputter coating a substrate comprising: providing a gas atmosphere below 10 microns of mercury pressure; ionizing a portion of said gas atmosphere to a plasma; providing a substrate adjacent to said plasma within said atmosphere; disposing between said plasma and substrate a target; maintaining said target at a negative potential between 300 and 2,000 volts and providing electric field lines between said plasma and the surface of said target, said electrical field lines having a substantial length thereof extending parallel to the shortest distance between said substrate and said plasma.

9. The process of claim 8 and including the step of rotating said substrate with respect to said target.

10. Apparatus for sputter-coating comprising: a sealed chamber, means for exciting low pressure gas in the chamber into an ionized plasma, means for maintaining gas within the chamber at sufficiently low pressure to permit the plasma formation; means for supporting a substrate within the chamber adjacent the plasma, a target grid of a coating material between the plasma and the substrate, means for maintaining the target at a negative electrical potential with respect to the plasma, and means disposed between the plasma and the target for directing electric field lines to a surface of the target.

References Cited UNITED STATES PATENTS 3,361,659 1/1968 Bertelsen 204298 3,458,426 7/1969 Rausch ct al. 204298 3,471,396 10/1969 Davidse 204298 3,501,393 3/1970 Wehner et al 204-298 3,526,584 9/1970 Shaw 204298 3,540,993 11/1970 Wurm et al. 204298 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner US. Cl. X.R. 204298 

